Known hot-dip coating systems for hot-dipping continuous metal strip often include a melting bath vessel, a snout that opens in the melting bath vessel and is configured to introduce a metal strip that is heated in a continuous furnace into the melting bath in a protective gas. They further include a deflecting roller disposed in the melting bath vessel and configured to deflect the metal strip that enters the melting bath, in a direction pointing out of the melting bath. In the case of prior art hot-dip coating installations, slag which may lead to defects in the coating of the metal strip accumulates on the surface of the molten metal within the snout. During the dipping of the strip, the slag is carried along by the strip and, for example, locations with poor adhesion arise due to slag inclusions and imperfections (uncoated locations) in the coating.
In order to prevent accumulation of slag on the melting bath surface within the snout, JP 04-120258 A proposes, inter alia, to produce within the dipped snout a flow directed counter to the running direction of the metal strip on both sides of the metal strip and, on the melting bath surface, a flow which is directed away from the metal strip and runs in the direction of entry of the metal strip into the melting bath.
An apparatus of the type mentioned at the beginning is known from EP 1 339 891 B1. The snout here is extended, on the dipped lower part thereof, on each side of the metal strip through an inner wall which is oriented toward the surface of the liquid seal bounded by the snout and the upper edge of which lies below said surface. Said inner walls together with the wall of the snout define two outflow spaces for the liquid metal. A pump is connected to the two outflow spaces via suction lines in order to keep the liquid metal level in said spaces to a level below the surface of the liquid seal and therefore to bring about a natural runoff of the liquid metal from said surface to the outflow spaces. For this purpose, the liquid metal level in said outflow spaces is detected and is kept to a level below the surface of the liquid seal in such a manner that the drop height of the liquid metal in the outflow spaces is greater than 50 mm in order to prevent buoyancy of the metal oxide particles and the intermetallic compounds counter to the runoff direction of the liquid metal. In order to make it possible to detect the liquid metal level in the outflow spaces, a reservoir in the form of a container which is open at the top is arranged outside the nozzle, said reservoir being connected via a pipeline to the lower region of each of the outflow spaces, wherein, in each of the outflow spaces, the connection point of the suction line of the pump lies above the connection point of the pipeline connected to the reservoir. The reservoir forms a liquid metal buffer capacity for each of the outflow spaces. In other words, the reservoir together with the outflow spaces forms, via the pipeline, a system of communicating pipes in which the liquid metal level is typically at the same height in each case. The reservoir is equipped here with a liquid metal level detector.
In the case of the apparatus known from EP 1 339 891 B1, considerable difficulties should be expected in industrial use. This is because there may be a shortfall in the required drop height of the liquid metal in the outflow spaces because of necessary snout movements or unavoidable fluctuations of the melting bath surface, which interferes with the outflow of slag directed away from the metal strip and, accordingly, may result in surface defects on the dip-coated metal strip.
The change in the position of the strip in the snout is an important requirement for surface-finished flat steel products. An optimum running of the strip through the melting bath and the blowout jets arranged above same can frequently be realized only by means of an adjustment of the dipped deflecting roller. Furthermore, the proposed solution of level regulation in the outflow spaces by means of reservoir and liquid metal level detector is susceptible to malfunction in industrial use since a considerable formation of slag occurs within the reservoir. The cleaning activity required for removing the slag from the reservoir is unsatisfactory from the point of view of working safety.